Nowadays, flat panels have been widely employed in color cathode ray tubes (CRTs) intended for use in television sets, computer monitors or the like, replacing the conventional spherical/parabolic panels, for enhanced visibility and reduction of glare due to reflection of external light.
Referring to FIG. 1, there is shown a schematic cross sectional view of a formed type flat panel for use in CRTs. The formed type flat panel 10 has a faceplate portion 12, a skirt portion 14 extending backwards from a periphery of the faceplate portion 12 and connected to a funnel (not shown), and a corner portion 16 (or blend round portion) integrally joining the periphery portion of the faceplate portion 12 with the skirt portion 14. The faceplate portion 12 is provided with a virtually flat external surface 12b and an inner surface 12a possessing a curvature considerably greater than that of the external surface 12b. And fixed on the skirt portion 14 by stud pins is a formed mask (not shown).
In contrast to the formed type flat panel, a tension type flat panel made of tinted glass uses a tension mask as a shadow mask. The tension type flat panel has a low wedge rate. That is, both external and inner surfaces are virtually flat, thus difference in thickness between a center portion and periphery portion of a faceplate portion thereof is considered negligible.
The formed type flat panel, however, has high wedge rate, and thus there exists a non-uniformity in light transmittance (or absorption) between the periphery portion and the central portion thereof. Consequently, it further leads to non-uniform brightness of image displayed thereon. To overcome such non-uniformity in brightness, intensity of brightness is increased over the entire faceplate portion of the formed type flat panel by making the formed type flat panel with clear glass, and in addition, coating the external surface of the faceplate portion in a manner that the high light absorption takes place in the central portion of the formed type flat panel while low absorption takes place in the periphery portion thereof.
Such a method described above, however, has some drawbacks as follows. First, the additional panel coating process incorporated in a procedure for fabricating the formed type flat panel inevitably raises the manufacturing cost.
Moreover, production of clear glass for clear formed type flat panels required in some applications alternately shares the glass melting furnace used for producing tinted glass gob for tinted formed type flat panels. Accordingly, such sharing process suffers from a tank color conversion loss, which occurs during a transition from one process to another, i.e., switching from a process for making clear glass gob for clear formed type flat panels to a process for making tinted glass gob for tinted tension type flat panels or vice versa. In practice, it takes several days to make such transition resulting in a significant loss of productivity.
Various efforts in making the formed type flat panel with the tinted glass have been attempted. However, the light transmittance of the formed type flat panel made of tinted glass is less than that of the formed type flat panel made of clear glass, accordingly arriving at an approach of making the thickness of the entire faceplate portion thinner than that of the faceplate portion made of clear glass so as to satisfy the tube makers' standard for brightness. While satisfying the brightness standard, such product made of tinted glass being thinner fails to meet the implosion resistance of the formed type flat panel.